A power plant engineer has many choices when selecting steel tubing materials for his condenser, feedwater
heater or balance-of-plant application. The wide variety of stainless steel choices available (ASTM lists
over 75 alloys) gives the engineer greater flexibility to choose the best candidate to meet budgetary
constraints and still provide the performance needed for the lifetime of the plant. Unfortunately, upset
conditions can be common in power generation, and these can result in premature unexpected failure of
steel tubing and piping materials. These may include differences in operation modes from design, changes in
water chemistry due to leaks in other parts of the system, corrosion from unexpected sources, impact of
improper lay-up practices, and the effect of corrosion product transport to other parts of the system. The
motivation to build modern combined-cycle power plants for the lowest cost per kilowatt has stretched the
envelope for materials performance.
This paper provides an overview on a number of factors known to cause failure of a tube or pipe material.
Knowing the limitations of material is crucial when making a selection for a specific application. This
paper helps to identify the factors that need to be considered when selecting a material. Properties
compared in this paper include corrosion resistance, stress corrosion cracking potential, thermal and
mechanical properties, erosion resistance, vibration potential, and temperature limitations. The property
comparison guides are intended to be quick tools to assist the user in selecting a cost-effective material
for a specific application.
Corrosion may be grouped into two broad categories, general corrosion and localized corrosion
accelerated by an electrochemical mechanism. The latter group can be divided into several well-known
General corrosion is the regular dissolution of surface metal. The two most common encountered are the
rusting of carbon steel and the wall thinning of copper alloys. General corrosion is normally not
catastrophic. With proper planning, a heat exchanger can be designed to accommodate general
corrosion, and in many instances, an alloy susceptible to this type of corrosion may be a cost-effective
design option. Heat exchanger designers commonly add a “corrosion allowance” to a high-pressure
carbon steel feedwater heater to allow for a 10 to 25 year lifetime.
Copper alloys are often chosen for condensing and BOP heat exchangers, and 25-year lifetimes are not
uncommon. In some applications, copper alloys are expected to slowly dissolve to maintain some
resistance to biofouling as the copper ion can be toxic to the microorganisms that attach to the tube wall.
Unfortunately, on the steam side of the tubing, copper transport to other locations due to this slow
dissolution may cause other problems. The copper can replate on turbine blades, resulting in a loss of
efficiency, or on boiler tubes, resulting in premature failures. Although the discharge values on the
cooling water side may be in the ppb concentration range, total copper metal discharge for a mediumsized condenser over the tubes’ lifetime can exceed several hundred thousand pounds per unit. In some
North American regions, high discharge levels have prevented the reuse of copper alloys in power plant
Electrochemically Driven Mechanisms
Several corrosion-related mechanisms are electrochemically driven, and these can be very unpredictable.
Therefore, they cannot be accommodated by design. These failure mechanisms usually have two
stages: an incubation or initiation period, and a propagation mode. The time of initiation can be very
unpredictable. It could happen in a few days or last for years. Once initiated, the second mode can occur
rather quickly, driven by the electropotential between the two regions. Conductivity of the water may be a
dominant factor. Higher conductivities allow higher current densities. Higher current densities are
proportionately related to metal removal rates.
Pitting corrosion is a highly localized attack that can result in through-wall penetration in very short
periods of time. Failures may occur in less than four weeks. Once a pit is initiated, the environment in
the pit is usually more aggressive than the bulk solution because of the pit’s stagnant nature. Even if the
bulk solution has a neutral or basic pH, the pH in a pit can drop below two. When this occurs, the surface
inside the pit becomes active. The potential difference between the pit and the more noble surrounding
area is the driver for the galvanic attack. As the surface area of the anode (pit) is small and the cathode
(the passive surface surrounding the pit) is large, a very high current density in the pit is possible. This
drives the very high corrosion rates.
The most common cause of pitting of stainless steels in the power industry is chlorides. Several alloying
elements, such as chromium, molybdenum, and nitrogen, promote chloride resistance in this group of
alloys. Not all have the same effect. By investigating the impact of each element, Rockel developed a
formula to determine the total stainless steel resistance to chloride pitting (1):
PREn = % Cr + 3.3 (% Mo) + 16 (N) (1)
PREn represents the “Pitting Resistance Equivalent” number. Using this formula, various stainless steels
can be ranked based upon their chemistry. In this formula, nitrogen is 16 times more effective and
molybdenum is 3.3 times more effective than chromium for chloride pitting resistance. The higher the
PREn, the more chloride resistance an alloy will have. It is interesting to note that nickel, a very common
stainless steel alloying element, has little or no effect on chloride pitting resistance. However, it does
have a profound impact in stress corrosion cracking which will be discussed later.
Crevice corrosion is very similar to pitting corrosion. However, since the tighter crevice allows higher
concentrations of corrosion products (less opportunity to flush with fresh water), it is more insidious than
pitting. This drives the pH lower. The end result is that crevice corrosion can happen at temperatures
30°-50° Centigrade lower than pitting in the same environment.
Crevice corrosion is commonly measured by the ASTM G 48 test. Kovach and Redmond evaluated a
large database of existing crevice corrosion data and compared it to the PREn number described earlier
(2). They developed relationships between the PREn and the G 48 critical crevice temperature (CCT)
and plotted the relationships. Figure 1 is a modified version to be used as a tool for comparing alloys and
determining maximum chloride levels.
Critical Crevice Temperature and Maximum Chloride Levels Versus PREn of Various Stainless
Ferritic stainless steels were found to have the highest CCT for a particular PREn, followed by the duplex
grade, and finally, the austenitics. Plotting the data for known alloys results in three separate almost
parallel correlations. After a typical or minimum chemistry is determined, the PREn can be calculated. To
compare the corrosion resistance of two or more alloys, a line is drawn vertically from the calculated
PREn for each alloy to the appropriate sloped line for the structure. The vertical line should stop at the
bottom line for austenitics, such as TP 304, TP 316, TP 317, 904L, S31254, and N08367. Duplex grades,
such as S32304, S32003, S33205, and S32750, fall on the center line. The ferritics, such as S44660 and
S44735, follow the top sloped line. From this intersection, a horizontal line should be drawn to the left
axis to determine an estimated CCT. A higher CCT indicates more corrosion resistance.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)